The Beijing Olympics are a huge occasion for China. Ever since the opium wars, the country has experienced what it describes as a "century of humiliation". Extraordinarily, the handover of Hong Kong in 1997 was its first major foreign policy success since the early 19th century.

Western countries are thoroughly accustomed to being the centre of global attention, which they have come to regard as their natural birthright. Not so China. It was thwarted in its attempt to hold the 2000 Olympics, which, as a result of American-led pressure, was awarded to Sydney. For China, therefore, the 2008 Olympics assume a huge importance as its first opportunity to command the global stage. The fact that the games also coincide with China's emergence as a global power only serves to enhance their significance. These Olympics, not surprisingly, have been long in the planning, with nothing left to chance.

But the global spotlight not only provides the Chinese government with an opportunity to show its wares to the world: it also offers those with a grievance against the government to do exactly the same. The fact that the games symbolically mark China's global "coming out" only serves to make them even more of a target for opposition causes.

The unrest in Tibet, then, is hardly unexpected. It would appear to have been sparked by a march of Buddhist monks to coincide with the 49th anniversary of China's military intervention in the autonomous region. With significant numbers dead - reports vary from the official Chinese version of 10 to as many as 100 or more - this is exactly the kind of event that the Chinese authorities have been dreading.

The other main attack on China in recent months has been for its policy on Darfur. Whatever the criticisms on this score, and whatever the future may hold, Chinese policy in Africa is certainly no worse than that of the west, and, historically speaking, is hugely better than the latter's miserable legacy. Tibet, on the other hand, raises much more troubling issues for China's standing in the world and how it is perceived by others.

The question is not whether Tibet should be independent but the extent of the autonomy that it is allowed. Tibet has been firmly ensconced as part of the Chinese empire since the Qing dynasty's military intervention in Tibet in the early 18th century. The Qing was responsible for a huge westward expansion of Chinese territory, adding lands populated by peoples, albeit relatively small in number, who had no natural affinity with the Chinese. One of the unique features of China is that, notwithstanding the fact that it has a population of 1.3 billion, around 92% regard themselves as Han Chinese. This is quite different from the world's other most populous countries, such as India, the US and Indonesia, which are ethnically diverse. China, of course, was once the same, but because it is at least two millennia old, it has experienced a remarkably long period of assimilation, melding and mixing.

The result is that China has little conception of difference. The Chinese think of themselves as one race. Their historical experience is one of slow and steady assimilation and absorption, with population settlement often a crucial instrument in pacification. In this light, the Han Chinese migration to Tibet and Xinjiang province in northwest China is nothing new: on the contrary it has been an age-old characteristic of Chinese expansion (a large majority of those who now live in Mongolia and Manchuria, for instance, are Han).

Tibet and Xinjiang, however, are distinguished by two important differences from other Chinese regions and provinces. First, in both cases their populations are ethnically very distinct from the Han Chinese. And second, their effective incorporation into China is relatively recent (though still more than two centuries ago). What is clear from the demonstrations and clashes in Lhasa and elsewhere is that the traditional Chinese policies of absorption have singularly failed to suppress the Tibetan sense of identity and desire for autonomy. Even though Tibetans have experienced major improvements in their living standards, this has not diminished their desire for religious and cultural freedom. It would seem, furthermore, that the huge wave of Han Chinese settlement has only served to heighten their sense of resentment and fear of loss.

Tibet and Xinjiang aside, it is unlikely that China will face anything like this kind of unrest in the next few months leading up to the Olympics. But events in Tibet have served to expose the achilles heel of modern China: its inability to recognise and respect ethnic difference within its own borders. As it emerges as a major global player in a world characterised by exactly such ethnic diversity, this seems destined to cast China in a rather more negative light, not least in the developing world.

· Martin Jacques is visiting research fellow at the Asia research centre, London School of Economics martinjacques@aol.com